Printed wiring boards (PWBs) are used as platforms for most electronic circuits. They serve as bases for electronic components such as integrated circuit chips, resistors, capacitors, and other familiar circuit elements. PWBs provide the actual physical base for mounting these electronic components, as well as the electrical connections required between the components. PWBs typically consist of a non-conductive dielectric layer, usually epoxy resin coated fiberglass, and a conductive layer, usually copper or some other metal.
To prepare a PWB, the conductive layer is subjected to a set of processes by which much of the conducting metal is removed from the non-conductive substrate, leaving behind a pattern of conductor that provides the proper electrical connections between the electronic components that are ultimately mounted to the PWB (i.e., the circuit pattern). In a photolithography process, the metal layer of the PWB is coated with a light-sensitive photoresist material. A mask is used to create a pattern in the photoresist layer by exposing only those parts of the conducting layer that will comprise the circuit pattern. The PWB is then subjected to ultraviolet developing light. The photoresist material that is on the exposed parts of the PWB, those parts not obscured by the mask, polymerizes on exposure to the ultraviolet light. Next, the mask is removed and the unpolymerized photoresist material is washed off, leaving exposed conductive layer in those parts blocked by the mask, but leaving a layer of polymerized photoresist material over the conductive layer of those parts that were not blocked by the mask. Finally, the PWB is etched in a process wherein the PWB is exposed to an etchant (e.g., cupric chloride, ammoniacal, or sulfuric peroxide). The etchant removes the exposed conductive layer, but leaves the parts of the conductive layer that are protected by the polymerized photoresist.
Modern PWBs often have rather dense circuitry. To accommodate the need for high density circuits, manufacturers have assembled layers of PWBs into multiple layer PWBs, typically with unique circuitry in each layer. These assemblies are typically comprised of alternating layers of PWBs and non-conducting dielectric layers such as epoxy resin coated fiberglass. Epoxy resin is typically the medium used to bind the layers into a single unit.
Multiple layer PWBs require electrical connections not only within each PWB, which is accomplished by photoresist etching as described above, but they also require electrical connections between layers in order to interconnect circuitry on different layers. For example, in a two-layered PWB, with two conductive layers separated by a layer of dielectric, the most efficient use of the PWB area is to allow circuits to span across the two conducting layers. This is typically achieved by drilling holes in the predetermined appropriate places on the multilayered PWB, then depositing a metal on the inside of the holes. The drilling step has become quite sophisticated, and often involves drilling extremely small holes on the order of a hundredth of an inch or smaller. After the drilled hole has been appropriately cleaned by removal of any material at the mouth of the hole, the hole is subjected to a process that makes it conductive. This may involve a non-electrolytic deposition of metal on the inside of the hole, followed by electroplating to the desired thickness, although many other alternative methods of depositing conductor in holes are also used.
When the holes in multi-layered PWBs have been rendered conductive, the deposited metal only coats the surface of the holes, thus effectively creating a hollow cylinder of conductive material. If the number of holes in a PWB is great enough, they can cause the PWB to become quite fragile even after the metal deposit. To reinforce the PWB, the holes may be filled with epoxy resin. Epoxy resin filling was commonly used for relatively large holes, i.e., holes with diameters of greater than about 0.020 inches, with relatively small aspect ratios (the ratio between the length of the hole to its diameter) where small aspect ratios are less than about 7. However, the modern trend has been towards smaller holes with greater aspect ratios, and these smaller holes have become almost impossibly difficult to fill with resin.
There is an ever present need in this art for better methods for filling holes in multi-layered PWBs. There is a particular need for methods for filling small holes with large aspect ratios in multi-layered PWBs.